1. Field of the Invention
The present invention relates to a sintered material containing as a main component silicon nitride (Si.sub.3 N.sub.4) having high strength and more particularly to improving the strength of a silicon nitride sintered material by the addition of aluminium oxide (Al.sub.2 O.sub.3) and yttrium oxide (Y.sub.2 O.sub.3) thereto.
2. Description of the Prior Art
Sintered substances containing Si.sub.3 N.sub.4 as a main component have been employed as parts in an automobile engine, a gas turbine, cutting tools, nuclear reactor structural materials and so on, since they possess excellent properties such as heat-resistance, high strength at high temperatures and a low coefficient of thermal expansion. With the intention of highly strengthening such substances and rendering them highly dense, additions of Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3, MgO and the compounds of Al.sub.2 O.sub.3 -MgO system have been used as a sintering aid. However, the strength of the sintered materials has not yet been raised to a satisfactory extent even by the addition of such sintering aids.